Lawrence Johnson offered me some excellent advice a while back which I
followed.  6 months later, I am very happy with the set up, and would
change nothing.

First, get a Bosch 40 amp relay.  There's room to attach it next to the
camper relay.  Run # 10 wire from the main battery positive terminal to
the new relay, and then from the switched terminal of the relay to the
positive on the optima.  This set up allows the aux battery to be charged
(after main battery charges), but prevents the main battery from being
drained when the engine is off.  There is a place to attach a ground cable
in the aux battery box.  Be sure to use heavy 40 amp fuses on the line
between the batteries.

I left the fridge set up as it was stock.  Never run it off of the battery
anyway.    I switched everything else over to the aux. batt.  I also ran a
hot wire to the fuse socket under the dash for the radio, etc, and plugged
it in to switch the radio,etc. over to the aux. batt.  Also have a trickle
charger and solar charger.  Works great.
steve

John Koloen wrote:

> I have decided to try the following to connect an auxiliary battery in
> my 87 Westy:
>
> Install an Optima battery in the aux box under the driver's seat.
> Connect the negative post of the aux battery to ground.
> Connect an 8 gauge wire to the post of the starter battery.
> Connect the other end of the 8 gauge wire to a switch to be mounted
> behind the driver's seat.
> Connect the switch to the positive post of the auxiliary battery.
> Connect the refrigerator wire to the auxiliary battery positive post.
> My intention is to leave the switch open while the engine is running
> and switch off when the engine is off so that the refrigerator runs
> off the auxiliary battery.
>
> I don't regard this to be an ideal solution but I'm planning on
> leaving on a 2-week trip in a few days and would like to have an
> auxiliary battery. (I'm refurbishing the propane tank and won't be
> finished until I return.) I've looked for a 75 amp relay to no avail.
> I've looked at isolators but I don't like the idea of installing it in
> the engine compartment. (I bought one from JC Whitney and it requires
> connecting the alternator leads to the isolator.) I've considered
> using the existing relay cube in the aux battery compartment but all
> of its jacks are used. Any comments are welcome as I am not an expert
> with electrical systems.